Acoustic wave filters such as surface acoustic wave (SAW) filters have been often used as filters used in communication devices such as mobile phones. The SAW filter includes a piezoelectric substrate and an IDT (Interdigital Transducer: comb-shaped electrode) located on the upper surface of the piezoelectric substrate. In addition, a duplexer and a module including filters may be installed in the communication device. The filter is required to have frequency characteristics achieving low-loss in the passband and high suppression outside the passband. The low-loss frequency characteristic enables to reduce the electrical power consumption of the communication device, and improve the speech quality. Japanese Patent Application Publication Nos. 9-284091 and 8-298430 disclose resonators that adjust the distance between the IDT and the reflector.
The loss in the passband is in a trade-off relationship with the degree of suppression outside the passband. For example, when the loss of a signal is reduced, the degree of suppression decreases.